MODEL RELEASED. DNA cloning. Researcher using a heat plate used to make identical copies (clones) of Deoxyribonucleic Acid (DNA) fragments during a Polymerase Chain Reaction (PCR). The technique uses the enzyme DNA polymerase to make single strands of DNA synthesise mirror copies of themselves. By repeatedly raising and lowering the temperature of the mixture until sufficient copies of the original fragment have been produced for analysis. PCR is useful where only a tiny amount of DNA can be isolated. This PCR machine is being used in a virology department.